Epitaxy by molecular beams or jets is a method for forming films of numerous materials on a substrate in ultravacuum or ultra-high vacuum. A molecular beam is obtained by producing a vapour pressure of the material in question in an enclosure or isothermal crucible having an opening.
The prior art discloses long crucibles, whose characteristics can be clearly defined. A cell of this type is relatively satisfactory when its operating temperature is sufficiently high and when the source material is in the liquid state. However, when using material supplying the necessary flux or flow by sublimation at low temperature (Te, As, P, Zn, Cd, S, Etc.), said cells suffer from the disadvantages referred to hereinafter.
Firstly, the material can condense at the mouth of the cell, bearing in mind that the latter is surrounded by liquid nitrogen-cooled panels. This condensation can extend to the plugging of the cell orifice.
The heat losses of the cell, essentially due to the radiation in ultravacuum are very small due to the low operating temperature. Therefore, if it is wished to reduce the cell temperature, a drop can only take place slowly after cutting off the heating means. Thus, there are heat regulating problems.
Moreover, it is difficult and even impossible to stove said cells, the latter operation consisting of heating the equipment to undergo epitaxy (ultravacuum enclosure and cell) to a temperature adequate for degassing the same. This operation generally takes place at a temperature of approximately 200.degree. to 250.degree. C., which can be adequate for emptying the cell charged or filled with a volatile material.
In addition, these cells do not make it possible to obtain stable flows from solid materials due to the inherent emission instability of a solid. This is due to the fact that the temperature of the material is not uniform. The material displaces, deforms and evaporates in puffs. To this is added the disadvantage of having projections of solid particles. Furthermore, the flow cannot be reproduced over a period of time due to the reduction of the charge.
Moreover, the diameter of the outlet ports used for such cells is sufficiently large to permit the entry of molecules from other cells, which are reflected by the furnace, by the substrate, or have not been trapped by the cooled surrouding panels. The interaction of these molecules with the charge of the cell can modify the evaporation level at a given temperature either by doping the source, or by forming a new compound on the surface of the charge, which leads to a variation of the emission surface and forms another cause of instability.
Finally, the inevitable degassing of the heating element is a by no means negligible source of pollution to the ultravacuum enclosure, the substrates to be coated and consequently the deposited products.